Solid Fuels

1. Coal gasification
 Coal gasification is the process of producing coal
gas, a type of syngas (mixture of carbon
monoxide (CO) and hydrogen (H2) gas) from coal.
3C (i.e., coal) + O2 + H2O → H2 + 3CO
 If, however, hydrogen is the desired end-product,
the coal gas (primarily the CO product) undergoes
the water gas shift reaction:
CO + H2O → CO2 + H2


2. Coal gasification
The chemistry of gasification is quite complex and is really accomplished through a
number of chemical reactions within the gasifier. Some of the major chemical
reactions are shown in the diagram.

3. Coal Gasification
 Coal gasification offers one of the most versatile and
clean ways to convert coal into electricity, hydrogen, and
other valuable energy products.
 Coal gasification electric power plants are now operating
commercially in the World
 Experts predict that coal gasification will be at the heart
of future generations of clean coal technology plants.

4. Coal Gasification
 Rather than burning coal directly, gasification (a thermo-
chemical process) breaks down coal - or virtually any
carbon-based feedstock - into combustible gaseous fuel.
 In a modern gasifier, coal is typically exposed to steam
and carefully controlled amounts of air or oxygen under
high temperatures and pressures.
 Under these conditions, molecules in coal break apart,
initiating chemical reactions that typically produce a
mixture of carbon monoxide, hydrogen and other
gaseous compounds.

5. Coal Gasification
The composition of the gas obtained from a gasifier
depends on a number of parameters such as:
 1. Fuel composition
 2. Gasifying medium
 3. Operating pressure
 4. Temperature
 5. Moisture content of the fuels
 6. Mode of bringing the reactants into contact inside
the gasifier, etc.

7. Gasifier Types
 Gasification processes are classified on the basis
of the method used to bring the coal into contact
with the gasifying medium (air or oxygen).
 The three principal commercial modes are
1. fixed-bed,
2. fluidized-bed, and
3. entrained-flow

8. Types of Gasifier
Coal (3-30 mm)
Gas
Fly Ash
Steam +Air / O2
Steam + O2
Coal (0.1 mm)
Gas
Coal (0.1 mm)
Steam + O2
Slag
Gas
Coal (1-5 mm)
Fly Ash
Moving Bed
(400-1,1000
C, 10-100 bar) Fluidised
(800-1,0500
C, 10-25 bar)
Entrained
(1,200-1,6000
C, 25-40 bar)
Steam +Air / O2

9. Temperature Profile of Gasifiers
FLUIDIZED BED GASIFIER
(800 – 10500C, 10 to 25 bar)
MOVING BED GASIFIER
(400-1100 0 C, 10 to 100 bar)
ENTRAINED FLOW GASIFIER
(1200-16000C, 25 to 80 bar)

10. Temperature Profile of Gasifiers

11. Fixed-Bed Gasifier
 In a fixed-bed gasifier, 1/4- to 2-in. coal is
supplied countercurrent to the gasifying medium.
 Coal moves slowly down (sometimes this type of
gasifier is called a moving-bed gasifier).
 Reaction zones typically consist of drying and
devolatilization, reduction, combustion, and ash
zones.
 In the drying and devolatilization zone, located
at the top of the gasifier, the entering coal is
heated and dried and devolatilization occurs.
 In the reduction / gasification zone, the
devolatilized coal is gasified by reactions with
steam and carbon dioxide.

12. Fixed-Bed Gasifier
 Heat exchanged with the entering
gasifying medium and fuel
 As a result both the ash and the
product gas leave at modest
temperature
 Fixed-bed gasifiers operating on
low-rank coals have exit
temperatures lower than 800 ◦F.
 Low oxidant requirements;
 Design modifications required for
handling caking coal;
 Limited ability to handle fines.

13. Fluidized-Bed Gasifier
 In this gasifier, coal with 1/8 to ¼ in. in
size enters the side of the reactor and
is kept suspended by the gasifying
medium.
 Similar to a fluidized-bed combustor,
mixing and heat transfer are rapid,
resulting in uniform composition and
temperature throughout the bed.
 The temperature is sustained below
the ash fusion temperature to avoid
clinker formation

14. Fluidized-Bed Gasifier
 Char particles entrained in the product gas are recovered
and recycled back into the gasifier via a cyclone.
 Acceptance of a wide range of solid feedstock
(including solid waste, wood, and high ash content coals);
 Uniform temperature;
 Moderate oxygen and steam requirements;
• Extensive char recycling

15. Entrained-Flow Gasifier
 Entrained-flow systems gasify pulverized fuel particles suspended in a
stream of oxygen (or air) and steam.
 Residence time in this type of gasifier is very short.
 Entrained-flow gasifiers generally use oxygen as the oxidant and
operate at high temperatures.
 Temperature is well above ash-slagging conditions, to ensure high
carbon conversion.
 Ash in the coal melts at the high operating temperature of the gasifier
and is removed as liquid slag
 The product gas and slag exit close to the reaction temperature

16. Entrained-Flow Gasifier
 . Entrained-flow gasifiers have the following characteristics :
 Ability to gasify all coals regardless of coal rank, caking
characteristics, or amount of coal fines,
 Feedstocks with lower ash contents are favored
 Uniform temperatures
 Very short fuel residence times in the gasifier;
 Very finely sized and homogenous solid fuel required;
 Relatively large oxidant requirements;
 High-temperature slagging operation
 Entrainment of some molten slag in the raw gas.

17. Entrained-Flow Gasifier
 Their use for biomass gasification is rather limited, as it
requires the fuel particles to be very fine (in the order of
80 to 100 µm).
 A number of manufacturers offer commercial entrained-
bed gasifiers for large-scale applications, such as
Texaco, Shell, and Koppers–Totzek.

18. Coal Gasification
Syngas H2

19. Pipeline gas from coal via Lurgi pressure gasification.
COAL
PREPARATION
OXYGEN-BLOWN
COAL
GASIFIERS
SCRUBBING
AND COOLING
CRUDE GAS
SHIFT CONVERSION
H2O+CO=H2+CO2
RECTISOL
GAS
PURIFICATION
METHANATION
3H2+CO=CH4+H2O
COMPRESSION
AND
DEHYDRATION
ELEMENTAL
SULFUR * 128 LTPD
CO2
H2S+CO2
GAS LIQUOR
25,303 TPD @ 8,920 Btu/lb
FEED
COAL
GAS LIQUOR SEPARATION
AND EFFLUENT
WATER TREATMENT
PIPELINE GAS
290 MMSCFD
@ 950 Btu/Scf
RECLAIMED WATER
TO PLANT USE
NAPHTHA
COAL TAR
COAL OIL
PHENOLS
AMMONIA
TOTAL LIQUIDS
8940 BBLS/day
155 tons/day
OILY
GAS
LIQUOR
SULFUR
PLANT
TARRY GAS LIQUOR
N2
OXYGEN
PLANT
AIR
COAL FINES
STEAM
OXYGEN
STEAM AND
POWER PLANT
ELECTRIC POWER
TO PLANT USE

20. System for In-Situ Gasification of
Coal
From Wiser, 2000
Inject air
and steam
Produce
low-Btu
gas
Chemical reactions

23. UCG – ADVANTAGES
 Energy obtainment from vast coal reserves which are not
commercially viable to mine
 Increase of worker safety as no mining operations involved
 Low environmental impact
 No atmospheric pollution
 No appreciable change in landscape
 Less subsidence than conventional mining
 Less Resettlement and Rehabilitation (R&R) issues
 No surface disposal of ash and coal tailings
 Potential GHG (greenhouse gas) reduction activity
(CO2 storage in cavities)

24. Selection of a Gasifier
 Type of bed
 Operating pressure
 Slagging, non-slagging
Lurgi (fixed bed, high P, non-slagging)
Koppers-Totzek (entrained bed, low P,
slagging)
Wellman-Galusha (fixed, low P, non-slagging)
Winkler (fluidized, low P, non-slagging)

28. Figure 7-11. The Texaco gasifier (also Shell, Eastman)